The stereo sound stage of a stereo audio system may be regarded as the apparent physical separation between two speakers in the stereo audio system. The sound stage of a stereo audio system is generally reflective of the physical size of the embodiment of the system. By way of example and not by way of limitation small, compact stereo recording playback systems, mobile telephone systems, portable sound playback systems and other similar systems all suffer from a generally small perceived sound stage. A manifestation of such a limitation is a perception by a user of a reduced separation of audio playback channels (e.g., right channel and left channel) during presentation of a stereo audio output.
It is known that subtracting some of a right channel signal from a left channel signal while subtracting some of a left channel signal from a right channel signal can expand the perceived sound stage outside the actual physical separation of the audio output units (e.g., loudspeakers) of a stereo audio system. This system handling approach may be referred to as “cross differencing”. Low-frequency acoustic signals behave substantially like general pressure changes in a typical room or space and are generally non-directional. It is common practice among audio system designers to provide for the cross signals to be filtered so that they are significantly reduced at lower frequencies (e.g., below 400 Hertz; Hz) so as to prevent cancellation of bass sound reproduction in the audio system.
Sound stage expansion techniques may be used in connection with video imaging, but voices may be perceived as being displaced from their sources. Such a result may prove to be confusing to viewers, so care must be exercised in employing a sound stage expansion system in connection with video systems.
Sound stage expanding techniques are intended to make an existing stereo sound stage seem wider than the actual physical span of the speakers producing the stereo sound presentation. However, sound stage expansion techniques are generally not themselves a creator of a stereo audio effect. Prior art employments of sound stage expansion have produced a significant variation of center audio images in comparison with left side and right side signals. This variation of center audio images is commonly manifested in voices and instruments being decreased in volume generally at center-stage as compared with left and right audio signals. A result is that listeners have difficulty in overlooking the center audio image variance and the effect of the sound stage expansion is not fully perceived as listeners are distracted by relatively louder left and right output signals compared to output signals appearing at the center. Prior art sound expansion apparatuses have not provided a means for adjusting center stage audio image or volume. Some prior art employments have also produced substantial increases in high frequency components of left and right output signals, or treble boost. As a result, listeners may be distracted by the treble boost and so do not fully perceive the effect of a sound stage expansion. Prior art sound stage expansion apparatuses have not provided a means of controlling treble boost.
Turning to FIG. 1, a schematic diagram of an example of a conventional system can be seen. System 10 includes a left amplifier unit 12 and a right amplifier unit 14. Left amplifier unit 12 has a non-inverting input terminal 20, an inverting input terminal 22 and an output terminal 24. A left signal providing unit 16 has a positive terminal 17 and a negative terminal 19. Positive terminal 17 is coupled with non-inverting input terminal 20. Negative terminal 19 is coupled with a ground terminal 25. A feedback network 30 including resistors R1, R2 couples output terminal 24 with inverting terminal 22, negative terminal 19 and ground terminal 25. Right amplifier unit 14 has a non-inverting input terminal 40, an inverting input terminal 42 and an output terminal 44. A right signal providing unit 36 has a positive terminal 37 and a negative terminal 39. Positive terminal 37 is coupled with non-inverting input terminal 40. Negative terminal 39 is coupled with a ground terminal 25. A feedback network 50 including resistors R3, R4 couples output terminal 44 with inverting terminal 42, negative terminal 39 and ground terminal 25. A cross differencing network 52 including cross differencing capacitor Cc and cross differencing resistor Rc couples a node or terminal 46 between resistors R1, R2 with a node or terminal 48 between resistors R3, R4.
In operation, amplifier unit 12 will force left feedback voltage VLf at inverting input terminal 22 to equal left input voltage VLi from left signal providing unit 16. This in effect applies an inverted left input voltage VLi to output terminal 44 via cross differencing network 52 and inverting input terminal 42 of right amplifier unit 14. The result is a reducing of right output voltage VRo at output terminal 44 by an amount related with an inverted left input signal VLi. Similarly, amplifier unit 14 will force right feedback voltage VRf at inverting input terminal 42 to equal right input voltage VRi from right signal providing unit 36. This in effect applies an inverted right input voltage VRi to output terminal 24 via cross differencing network 52 and inverting input terminal 22 of left amplifier unit 12. The result is a reducing of left output voltage VLo at output terminal 24 by an amount related with an inverted right input signal VRi. This cross differencing signal effects sound stage expansion using apparatus 10. However, cross differencing network 52 has a deleterious effect in that it increases gain for both of amplifier units 12, 14 above its characteristic frequency fc:
                                          f            c                    =                      1                          2              ⁢              π              ⁢                                                          ⁢                              R                c                            ⁢                              C                c                                                    ,                            (        1        )            where Rc is the value of resistor Rc in network 52 and Cc is the value of capacitor Cc in network 52. Increasing gain for amplifier units 12, 14 at frequencies higher than characteristic frequency fc is manifested as increased volume for higher frequency signals, such as treble audio output signals. As mentioned earlier herein, such variation of treble signals is distracting to listeners. The effect of the sound stage expansion is not fully perceived as listeners concentrate on louder treble signals to the left and right. Sound stage expansion apparatus 10 does not provide a means for adjusting high frequency response or center stage audio image or volume.
There is a need for an apparatus and method for effecting sound stage expansion that permits mitigating of center audio image variation and controls treble boost in sound presentation.